Prospects of bacterial-assisted remediation of metal-contaminated soils

Abstract

Industrial revolution resulted in plenty of contaminants in the environment. Several organic and inorganic pollutants have adversely affected soils and water resources, causing serious health issues in humans. Among inorganic contaminants heavy metals are of prime importance as they are nondegradable in the environment. Arsenic, cadmium, chromium, cobalt, copper, lead, mercury, selenium, zinc, and other metals originating from various point and nonpoint sources are contaminating natural resources. Elevated concentrations of poisonous metals are not only disturbing soil health and microbial ecology but also decreasing crop production and global food security. Entry of metal pollutants into the food chain is dangerous for human health. Serious efforts are needed to mitigate rising threats of metal contamination. Physical, chemical, and biological approaches can be used to remediate such type of pollutants. However, bioremediation is considered as a promising technique, being cost effective and environment friendly with minimum adverse effects, esthetic advantages, and long-term applicability. Phytoremediation is a type of bioremediation to remove toxic metals from soil through hyperaccumulation or phytostabilization in plant cells. Generally, higher contents of toxic metals in soil and water result in more uptake by roots and more translocation toward shoots, causing interference in metabolism and reduced growth. Successful phytoremediation is limited to the plant types, tolerance to the high metal concentrations, accumulation rate, growth rate, adaptability, and biomass production. Metal-tolerant bacteria can help plant to tolerate metal stress via different mechanisms involved including production of different hormones such as auxins, cytokinin, and gibberellic acid or suppressing stress-induced enzymes such as plant ethylene level. This chapter reviews possible interactions between plant and bacteria to make situations more conducive for remediation of metal-contaminated soil. The chapter also covers different strategies/mechanisms adopted by plants and bacteria to mitigate toxic effect of metals on plant growth in metal-contaminated soils.